Direct separation of Zn from dilute aqueous solutions for isotope composition determination using multi-collector ICP-MS

We report here a two-step Zn chemical separation protocol, which allows dilute aqueous samples with low Zn concentrations to be directly loaded onto the chromatography columns, without any preliminary evaporation. This method is particularly suited for river and rain water samples with Zn concentrations between 0.1 and 120 μg/l. We present the different steps of the protocol on Chelex-100 and AG 1-X4 resins (Bio-Rad) with varying eluants under different matrix conditions. The method results in acceptable procedural blanks and quantitative yields of Zn. The mass bias during isotope measurements by the Neptune Thermo® MC-ICP-MS was corrected using empirical external normalization (EEN) with Cu as the internal dopant. We additionally evaluate the sensitivity of the measurement method to instrumental and analytical conditions as tuning, solution pH, Zn concentration, Zn/Cu ratio, temperature, and trace matrix addition. Both the stable introduction system (SIS, double Scott cyclonic spray chamber) and the Apex HF (ESI) were tested for Zn isotope measurement and the Apex was preferred for its higher sensitivity and better stability. External δ66Zn reproducibility for measurements of the standard solution (calibrated over two years) and natural water samples was 0.04‰ (2σ).